1. Field of the Invention
This invention relates to a leakage flux flaw detection method and apparatus for detecting flaws in ferromagnetic materials such as alloys containing iron, cobalt or nickel, for example, by magnetizing the ferromagnetic material and utilizing magnetic flux leakage from a flaw portion to detect flaws.
2. Prior Art Statement
Magnetic flaw testing is a non-destructive method of testing for flaws in ferromagnetic materials such as alloys of iron, cobalt or nickel.
Leakage flux flaw detection is a typical example of a magnetic flaw test method in which a test material is magnetized and flaws are detected by detecting flux leakage from the flawed portion, and which is used in magnaflux flaw detection and magnetic recording flaw detection.
The conventional magnaflux flaw detection method will now be described with reference to FIG. 1. When magnetic powder 102, such as iron particles, for example, is scattered on the test surface 110 of a test material 101 that is magnetized using a magnetizer or the like, the magnetic powder 102 will adhere to the test surface 110 of the test material 101 if the test surface 110 or surface layer portion of the test material 101 is free of flaws, since the strength of the magnetic field will be the same over the whole surface portion of the material and there is no leakage of magnetic flux (FIG. 1 (a)).
However, when the material has a flaw such as the flaw 103 in the material 101a shown in FIG. 2, an electric field is formed across the groove 104 of the flaw 103, with the wall 105 at one side of the groove forming the negative pole and the wall 106 at the other side forming the positive pole, and part of the magnetic flux 107 that connects the two sides forms a leakage magnetic flux 107a from the test surface 110 of the material 101a. This causes a concentration of the magnetic powder 102 along the lines of leakage flux around the flaw 103 portion, forming a magnetic powder pattern 108 on the test surface 110 (FIG. 1 (b)). Thus, the shape and extent of a flaw such as the flaw 103, and its depth, is estimated by the presence or absence of such a magnetic powder pattern 108, and the shape, size and height of the pattern.
A feature of the magnaflux flaw detection method is that it does not require elaborate equipment and can be readily carried out by anyone. The method has a number of drawbacks, however. For example, the method involves a manual operation and therefore cannot be carried out unattended and the results depend on the technical proficiency of the person performing the examination, very small flaws are often overlooked owing to the equivocal nature of the magnetic powder pattern and the fact that recording it is not possible; and objective, quantitative evaluations and evaluations based on changes over time are also difficult.
In the conventional magnetic recording flaw detection method, the magnetic head has a large gap and, as shown in FIG. 3, with the magnetic tape 111 or other such recording medium laid on the test surface 110 of the test material 101, the material 101a is magnetized by means of the magnetizer 100 (FIG. 3 (a)) and leakage flux 107a from the flaw 103 portion of the material 101a is recorded on the magnetic layer 111a of the magnetic tape 111. Thus the degree of magnetization of the portion of the magnetic layer 111a through which the leakage flux 107a passes is made to correspond to the general direction of the leakage flux 107a (FIG. 3 (b)), and after the recording the magnetic tape 111 is run past magnetic head 112 to convert the magnetization orientation and intensity to corresponding induction voltages (FIG. 3 (c)) and the voltage signal waveform thus obtained is analyzed to evaluate the flaw 103.
Since the magnetic recording flaw detection method can be automated it can be performed unattended, and the ability to make a recording makes it possible to make objective, quantitative evaluations and evaluations of changes over time.
However, a problem with the conventional magnetic recording flaw detection method is that the magnetic recording head has a large gap and the magnetic force of the leakage flux 107a with respect to the magnetic layer 111a is small, so that the leakage flux 107a is not clearly recorded on the magnetic tape 111. At even a minute distance from the test surface 110 there is a rapid attenuation of the leakage flux 107a, so it is therefore necessary to keep the magnetic tape 111 lift-off as small as possible. Means of achieving this include placing a rubber sheet on the magnetic tape 111, but when a flaw is very small or the material is very thin or is coated, the signals obtained may be so weak as to be indistinguishable from noise, in which case it is, for practical purposes, impossible to conduct the test.
There are other magnetic flaw detection methods. However, if a magnetic sensor is used the lift-off produced by the thickness of the sensor also makes it difficult to detect small flaws and flaws in thin or coated materials.